This invention concerns coordinate measuring machines.
Coordinate measuring machines (referred to as CMM's) are well known and in widespread industrial use. A CMM consists of a probe supported for movement along three orthogonal axes above a table on which an object to be measured is supported. The extent of movement of the probe along each axis is measured precisely by transducer devices generating corresponding electrical signals, such as Moire' fringe gratings and photocell arrangements or laser beam interferometers. This arrangement allows measurements to be made by movement of the probe tip between points of interest on an object to be measured, by processing of the electrical signals to calculate the distances between points from the extent of movement of the probe.
It is important that the probe be supported so as to not generate frictional or misalignment forces tending to deflect the support members, particularly in a nonrepeatable fashion, as the accuracy of measurement depends on precise correspondence between the probe tip movement and movement of the structure carrying the probe during each movement.
One form of CMM comprises a vertically extending probe mounted for movement along a vertical axis (here referred to as the Z-axis) on a first or probe carriage. The probe carriage is itself in turn mounted for movement in a first horizontal direction, here referred to as the X-axis, on a horizontal beam extending across and above the table.
The horizontal beam forms a part of a second carriage supported for movement along a second horizontal axis (here referred to as the Y-axis).
The Y-axis carriage consists of a pair of uprights straddling the table and supported on air bearings and fixed ways.
In an effort to improve the rigidity of the Y-axis carriage, a lower cross member has heretofore been added, connecting the lower ends of the uprights. See U.S. Pat. Nos. 4,594,791 and 4,630,374 for examples of such designs.
The table has usually been constructed of a rigid material such as steel to avoid distortions caused by the weight of heavy objects to be measured, while the carriages are constructed of a lightweight material such as aluminum to be as lightweight as possible to improve performance when servo drives are employed. The table is supported on a base, the base forming the ways on which the Y-axis carriage moves, as in the above U.S. patents. The base is often also constructed of a lightweight material such as aluminum, different from that of the table.
The Y-axis carriage support and guide bearings have heretofore been provided on the sides of the base as shown in the above cited U.S. patents. This leads to difficulties, particularly if diverse materials are used in the carriage and base due to differences in the coefficient of thermal expansion. Even if the same material is used, differences in mass or in exposure to transient conditions can create transitory differences in thermal growth. The guide bearings engage widely spaced vertical way surfaces in these described prior designs, the wide spacing accentuating differences in thermal growth such that loss of the necessary air gap can result.
The guide ways must be very accurately matched to each other to insure accuracy, and with guide surfaces on either side of the base, these must usually be machined while fixed or integral with the base itself necessitating a large machining apparatus with increased manufacturing costs.
Where four bearings are used in each set, the machining accuracy requirements are particularly acute, since the way surfaces must be precisely located in parallel planes to prevent interferences.
U.S. Pat. No. 4,610,089 describes a center guide rail design intended to alleviate this problem by using a central rail having vertical guide surfaces thereon and affixed to the base or the underside of the table.
U.S. Pat. Nos. 4,852,267 and 4,763,420 show X-axis carriages, lacking a lower cross member, which are guided with an asymmetric guide surface arrangement, in which vertical guide surfaces are located on one side of the table or base only, and horizontal support surfaces only are formed on the opposite ways.
USSR patent 621955 shows an asymmetric bearing arrangement for a Y-axis carriage which also has a lower cross member passing beneath the table.
It is the object of the present invention to provide a lower cost construction coordinate measuring machine while ensuring adequate accuracy and freedom from thermal growth problems.